1. Field of the Invention
The present invention relates to a novel method for producing a reflective electrochromic device.
1. Related Background Art
A phenomenon of reversible color generation by reversible electrolytic oxidation or reduction under application of a voltage is called electrochromism. There have been attempts, for more than 20 years, to produce electrochromic devices (ECD) having a thin film of an electrochromic (EC) material, capable of showing such phenomenon, sandwiched between a pair of electrode layers and capable of generating and erasing color through the control of a voltage applied across said electrode layer, and to utilize such electrochromic devices as a light intensity controlling element, such as a nonglare mirror, or as a 7-segment numeral display element.
For example, the U.S. Pat. Nos. 3,521,941 and 3,829,196 disclose a totally solid-state ECD composed of a laminate structure of a transparent electrode layer, a thin film of tungsten trioxide and an insulating film, for example composed of silicon dioxide, formed on a glass substrate. Under the application of a coloring voltage Vc, the ECD generates blue color in the thin film of tungsten trioxide (WO.sub.3), which is reduced to colorless under the application of an erasing voltage Vb of opposite polarity.
Although the mechanism of coloring and erasing has not been fully clarified, a small amount of water present in the WO.sub.3 film and the insulating film (ion conductive layer) is presumed to govern the color generation and erasure of WO.sub.3. The color generating reaction is conjectured as follows: EQU H.sub.2 O.fwdarw.H.sup.+ +OH.sup.-
(WO.sub.3 film at the cathode side) ##STR1## (insulating film at the anode side) EQU 2OH.sup.- .fwdarw.H.sub.2 O+1/2O.sub.2 .uparw.+e.sup.-
As will be understood from the foregoing reaction formulas, WO.sub.3 is a reductive coloring EC material which is colored by reduction. Also MoO.sub.3 shows a similar behavior.
However, the reduction is always accompanied by an oxidation reaction. In the above-explained case, an oxidation reaction: EQU 2OH.sup.- .fwdarw.H.sub.2 O+1/2O.sub.2 .uparw.+e.sup.-
takes place in the system of ECD. In this manner such ECD has been associated with the drawbacks of consumption of the contained water, in the coloring reaction, by an undesirable side reaction giving rise to the generation of oxygen gas, and indispensable water replenishment from the air for repeating the coloring process, as the water is not generated in the erasing reaction. In the ECD of this type, particularly due to the latter reason, the reproducibility of coloring process is easily affected by the water in the air.
On the other hand, U.S. Pat. No. 4,350,414 discloses a totally solid-state ECD which is capable of repeating the coloring and erasure without the water replenishment from the outside and in which the color density in repeating coloring operations is not affected by peripheral conditions, since the water is generated in the erasing reaction in an amount the same as that consumed in the coloring reaction.
The ECD is typically composed of the following:
A: electrode layer PA0 B: reductive coloring EC layer such as WO.sub.3 or MoO.sub.3 ; PA0 C: ionic conductive layer such as SiO.sub.2 or Ta.sub.2 O.sub.3 ; PA0 D: oxidative coloring EC layer such as Cr.sub.2 O.sub.3, IrO.sub.x or Ir(OH).sub.y ; and PA0 E: electrode layer. PA0 A: electrode layer; PA0 B: reductive coloring EC layer; PA0 C: transparent ionic conductive layer; PA0 D.sub.0 : metallic iridium thin film; and PA0 E: electrode layer PA0 A: electrode layer; PA0 B: reductive coloring EC layer; PA0 C: ionic conductive layer; PA0 D: dispersion layer consisting of iridium oxide or hydroxide (disperse phase) and a dispersion medium; and PA0 E: electrode layer, PA0 A: reflective metallic electrode layer; PA0 D.sub.0 : layer containing metallic iridium or lower oxide thereof; PA0 C: ionic conductive layer PA0 B: reductive coloring electrochromic layer; PA0 E: transparent electrode layer; and PA0 S: transparent substrate,
At least one of the electrode layers A, E naturally has to be transparent. Either may be utilized as a reflective layer.
The ECD disclosed in the aforementioned U.S. Pat. No. 4,350,414, utilizes iridium hydroxide Ir(OH)y. As in ordinary hydroxides, iridium hydroxide can be regarded as a hydrate of iridium oxide IrO.sub.x. In the present text, therefore, an expression "oxide of iridium", "iridium oxide", or "IrO.sub.x " should be understood to include hydroxide or a mixture with hydroxide. These materials are the best oxidative coloring EC materials in ECD's ever reported.
An iridium hydroxide film, functioning as the oxidative coloring EC layer, can be prepared either by a method described in said U.S. Pat. No. 4,350,414, in which a metallic iridium film is at first formed by a thin film deposition method under vacuum such as vacuum evaporation, sputtering or ion plating, and is subsequently converted into an iridium oxide film by anodic oxidation in an electrolyte solution such as sulfuric acid; or a method described in U.S. Pat. No. 4,258,984, in which an iridium oxide film is directly formed in one step by reactive sputtering utilizing a metallic iridium target in an oxygen atmosphere; or a method described in Japanese Laid-open Patent Application 58-70215 in which the following 5-layered structure (ECD precursor) is prepared:
and an AC current is applied across the electrodes A, E in a subsequent step in a gas atmosphere containing water vapor, thereby converting the D.sub.0 layer (metallic iridium) into iridium oxide or iridium hydroxide.
However, in any of these methods, the D layer is composed solely of iridium oxide or hydroxide, and the ECD of U.S. Pat. No. 4,350,414 utilizing such D layer has been associated with poor resistance to high temperature.
In order to resolve this drawback, U.S. Pat. No. 4,652,090 proposes an ECD which has a 5-layered structure as proposed in U.S. Pat. No. 4,350,414 but in which the oxidative coloring EC layer is not composed of pure iridium oxide or hydroxide, but is composed of a dispersion thereof with a dispersing medium such as SnO.sub.2.
Such a dispersion layer can be prepared by one of the following three methods, namely (1) a method in which a dispersion layer composed of metallic iridium and a dispersing medium is at first prepared, and is then converted into a dispersion layer composed of iridium oxide or hydroxide and dispersing medium by oxidation of metallic iridium by (i) thermal oxidation or (ii) anodic oxidation; (2) a method of directly obtaining the dispersion layer in a step by non-reactive thin film deposition under vacuum, utilizing iridium oxide and dispersing medium as evaporation sources or targets; and (3) a method of directly obtaining the dispersion layer in one step by reactive thin film deposition under vacuum utilizing iridium oxide and dispersing medium as evaporation sources or targets.
In any case, iridium oxide or hydroxide exists in particles of molecular level or in superfine particles in the dispersion.
However the method (1)(i) utilizing thermal oxidation can never achieve complete oxidation of metallic iridium, so that the obtained dispersion layer is not completely transparent and is gray colored.
The method (1)(ii) employing anodic oxidation significantly degrades the production efficiency of ECD, and elevates the cost, due to the presence of a wet anodic oxidation step in contrast to other methods consisting solely of dry steps.
The method (2) utilizing iridium oxide as the evaporation source or the target is associated with the drawbacks of difficulty in the preparation of the evaporation source or the target, and of formation of coarse particles in the dispersion layer, due to sudden boiling in the course of manufacture.
The method (3) employing reactive deposition is associated with the drawbacks of a limited dynamic range of coloring and erasure in the obtained ECD and of poor reproducibility in performance among different production lots.